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Author: Publisher: ISBN: Category : Languages : en Pages : 12
Book Description
Over the decade 1996-2006, acoustic sources located off central California and north of Kauai transmitted to receivers distributed throughout the northeast and north central Pacific. Some of the observations included "shadow-zone arrivals", that appear at travel times aligned with the lower cusps of the acoustic time front predicted by ray calculations, but with the depth of the receiver lies well below the depths of the predicted cusps. Several models for the temperature and salinity in the North Pacific Ocean were obtained and processed to enable simulations of acoustic propagation for comparison to the observations. New tools were developed to manage the large size of the model output, to extract and construct the relevant acoustic properties from the model output, and to make the acoustic calculations. Computer codes using ray tracing and the parabolic equation to calculate acoustic properties were significantly developed. The acoustic data show that WOA05 is a better estimate of the time-mean hydrography than either the JPL-ECCO or the POP estimates, which proved incapable of reproducing the observed acoustic arrival patterns.
Author: Publisher: ISBN: Category : Languages : en Pages : 12
Book Description
Over the decade 1996-2006, acoustic sources located off central California and north of Kauai transmitted to receivers distributed throughout the northeast and north central Pacific. Some of the observations included "shadow-zone arrivals", that appear at travel times aligned with the lower cusps of the acoustic time front predicted by ray calculations, but with the depth of the receiver lies well below the depths of the predicted cusps. Several models for the temperature and salinity in the North Pacific Ocean were obtained and processed to enable simulations of acoustic propagation for comparison to the observations. New tools were developed to manage the large size of the model output, to extract and construct the relevant acoustic properties from the model output, and to make the acoustic calculations. Computer codes using ray tracing and the parabolic equation to calculate acoustic properties were significantly developed. The acoustic data show that WOA05 is a better estimate of the time-mean hydrography than either the JPL-ECCO or the POP estimates, which proved incapable of reproducing the observed acoustic arrival patterns.
Author: Lora J. Van Uffelen Publisher: ISBN: Category : Languages : en Pages : 89
Book Description
Deep acoustic shadow-zone arrivals were first observed on horizontal, bottom-mounted receiving arrays in the North Pacific Ocean in the late 1990s. These receptions revealed significant acoustic energy penetrating an estimated 500-1000 m into geometric shadow zones below cusps (caustics) of predicted timefronts, much more than predicted by diffraction theory. Two vertical line array receivers deployed in close proximity in the North Pacific as part of the SPICEX experiment, together virtually spanning the water column, show the vertical structure of the shadow-zone arrivals for transmissions from broadband 250-Hz acoustic sources moored at the sound channel axis (750 m) and slightly above the surface conjugate depth (3000 m) at ranges of 500 and 1000 km. Chapter II compares a daily average of acoustic timefronts with parabolic equation simulations based on a sound-speed profile measured nearly concurrently with the acoustic data acquisition. Simulations incorporating a range-independent sound-speed profile confirmed the presence of deep shadow-zone arrivals. Receptions from off-axis sources also revealed acoustic energy scattering back up toward the axis at the end of the timefront, referred to as axial shadow-zone arrivals. Simulations incorporating sound-speed fluctuations consistent with the Garrett-Munk internal-wave energy spectrum at full strength accurately predict the vertical extent of and energy contained in both axial and deep shadow-zone arrivals. Chapter III extends the analysis to include acoustic receptions from June to November 2004. Incoherent monthly averages of acoustic timefronts indicate that lower cusps associated with acoustic rays with shallow upper turning points (UTPs), where sound-speed structure is most variable and seasonally dependent, deepen from June to October as the summer thermocline develops. Surface-reflected rays, or those with near-surface UTPs, exhibit less scattering due to internal waves than in later months, when the UTP deepens. Data collected in November exhibits dramatically more vertical extension than previous months. The depth to which the timefronts extend as the seasons change is a complex combination of deterministic changes in the depths of the lower cusps as the range-average profiles evolve and of the amount of scattering, which depends on the depths of the UTPs and the mean vertical gradients at those depths.
Author: Ralph A. Stephen Publisher: ISBN: Category : Kinematics Languages : en Pages : 94
Book Description
These notes provide supporting information for a JASA (Journal of the Acoustical Society of America) LttE (Letter to the Editor) manuscript, "Deep seafloor arrivals: A new class of arrivals in long-range ocean acoustic propagation" (Stephen et al., submitted). It addresses five issues raised by the co-authors: 1) incorrect processing for the time-compressed traces at T2300 and T3200 that appeared in an early version of the LttE (T2300, T3200 ... refer to transmissions at 2300, 3200km etc from the DVLA (Deep Vertical Line Array)), 2) processing issues, including the trade-offs between coherent and incoherent stacking and corrections for the effects of moving sources and receivers and tidal currents (Doppler), 4) the distinction between "deep shadow zone arrivals", which occur below the turning points in Parabolic Equation (PE) models, and "deep seafloor arrivals", which appear dominantly on the Ocean Bottom Seismometer (OBS) but are either very weak or absent on the deepest element in the DVLA and do not coincide with turning points in the PE model (some of these OBS late arrivals occur after the finale region), 4) the role of surface-reflected bottom-reflected (SRBR) paths in explaining the late arriving energy, and 5) generally reconciling the OBS analysis with work by other North Pacific Acoustic Laboratory (NPAL) investigators and Dushaw et al (1999).
Author: Publisher: ISBN: Category : Languages : en Pages : 12
Book Description
The North Pacific Acoustic Laboratory (NPAL) program is intended to improve our understanding of (i) the basic physics of low-frequency, broadband propagation in deep water, including the effects of oceanographic variability on signal stability and coherence, (ii) the structure of the ambient noise field in deep water at low frequencies, and (iii) the extent to which acoustic methods, together with other measurements and coupled with ocean modeling, can yield estimates of the time-evolving ocean state useful for acoustic predictions. The goal is to determine the fundamental limits to signal processing in deep water imposed by ocean processes, enabling advanced signal processing techniques to capitalize on the three-dimensional character of the sound and noise fields.
Author: Ralph A. Stephen Publisher: ISBN: Category : Ambient sounds Languages : en Pages : 88
Book Description
This report gives an overview of the analysis that was done on Deep Seafloor Arrivals since they were initially presented in Stephen et al (2009). All of the NPAL04/LOAPEX (North Pacific Acoustic Laboratory, 2004/ Long Range Ocean Acoustic Propagation Experiment) data on three ocean bottom seismometers (OBSs) at ~5,000m depth and the deepest element of the deep vertical line array (DVLA) at 4250m depth has been analyzed. A distinctive pattern of late arrivals was observed on the three OBSs for transmissions from T500 to T2300. The delays of these arrivals with respect to the parabolic equation predicted (PEP) path were the same for all ranges from 500 to 2300km, indicating that the delay was introduced near the receivers. At 500km range the same arrival was observed throughout the water column on the DVLA. We show that arrivals in this pattern converted from a PEP path to a bottom-diffracted surface reflected (BDSR) path at an off-geodesic seamount.
Author: Ralph A. Stephen Publisher: ISBN: Category : North Pacific Ocean Languages : en Pages : 241
Book Description
The Ocean Bottom Seismometer Augmentation in the North Pacific Experiment (OBSANP, June-July, 2013, R/V Melville) addresses the coherence and depth dependence of deep-water ambient noise and signals. During the 2004 NPAL Experiment in the North Pacific Ocean, in addition to predicted ocean acoustic arrivals and deep shadow zone arrivals, we observed "deep seafloor arrivals" (DSFA) that were dominant on the seafloor Ocean Bottom Seismometer (OBS) (at about 5000m depth) but were absent or very weak on the Distributed Vertical Line Array (DVLA) (above 4250m depth). At least a subset of these arrivals correspond to bottomdiffracted surface-reflected (BDSR) paths from an out-of-plane seamount. BDSR arrivals are present throughout the water column, but at depths above the conjugate depth are obscured by ambient noise and PE predicted arrivals. On the 2004 NPAL/LOAPEX experiment BDSR paths yielded the largest amplitude seafloor arrivals for ranges from 500 to 3200km. The OBSANP experiment tests the hypothesis that BDSR paths contribute to the arrival structure on the deep seafloor even at short ranges (from near zero to 4-1/2CZ). The OBSANP cruise had three major research goals: a) identification and analysis of DSFA and BDSR arrivals occurring at short (1/2CZ) ranges in the 50 to 400Hz band, b) analysis of deep sea ambient noise in the band 0.03 to 80Hz, and c) analysis of the frequency dependence of BR and SRBR paths. On OBSANP we deployed a 32 element VLA from 12 to 1000m above the seafloor, eight short-period OBSs and four long-period OBSs and carried out a 15day transmission program using a J15-3 acoustic source.
Author: Publisher: ISBN: Category : Languages : en Pages : 8
Book Description
Analysis of data from the NPAL04 experiments has shown that there are three principal types of arrival in long-range ocean acoustic propagation: 1) arrivals predicted by PE model calculations (RR, RSR, RBR, SRBR ray paths and the mode-like finale region), 2) deep shadow zone arrivals arising from the spread of energy below PE predicted turning points (caustics) and attributed to internal waves (Van Uffelen et al., 2009), and 3) deep seafloor arrivals which can be the largest arrivals observed on seafloor receivers and do not correspond to turning points or any other features in the PE predicted path (Stephen et al., 2009; Stephen et al., 2008). Coda and "bottom junk" have been ubiquitously observed on acoustic receptions on seafloor receivers and are traditionally attributed to incoherent reverberation and scattering. The OBS data on NPAL04 has shown that there are robust, coherent, discrete arrivals that contribute to the coda and bottom junk. The long-term goal of this project is to understand the role of bottom interaction in long-range ocean acoustic propagation. At the moment we do not understand the physical mechanisms responsible for the deep seafloor arrivals and we do not understand the implications for seafloor receptions in shallower water.
Author: D. Lee Publisher: Elsevier ISBN: 1483295699 Category : Science Languages : en Pages : 134
Book Description
A concise guide to the theory and application of numerical methods for predicting ocean acoustic propagation, also providing an introduction to numerical methods, with an overview of those methods presently in use. An in-depth development of the implicit-finite-difference technique is presented together with bench-mark test examples included to demonstrate its application to realistic ocean environments. Other applications include atmospheric acoustics, plasma physics, quantum mechanics, optics and seismology.
Author: Publisher: ISBN: Category : Languages : en Pages : 7
Book Description
The long-term goal of this research is to understand deep-water propagation, with particular emphasis placed on the passive quiet target detection problem. Focus is on the spatial/temporal structure of acoustic paths for moving sources and moving receivers. Propagation paths are separated into two classes: bottom interacting and refracted/surface-reflected (non-bottom interacting). This research seeks to understand the impact that mid-ocean variability (internal waves, mixed layer variability) and seafloor scattering have on the detection problem.
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Author: Lora J. Van Uffelen
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